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Register a new userThe magnetic field particle-hole excitation spectrum in doped graphene and in a standard two-dimensional electron gas
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The particle-hole excitation spectrum for doped graphene is calculated from the dynamical polarizability. We study the zero and finite magnetic field cases and compare them to the standard two-dimensional electron gas. The effects of electron-electron interaction are included within the random phase approximation. From the obtained polarizability, we study the screening effects and the collective excitations (plasmon, magneto-excitons, upper-hybrid mode and linear magneto-plasmons). We stress the differences with the usual 2DEG.
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We compute the single-particle states of a two-dimensional electron gas confined to the surface of a cylinder immersed in a magnetic field. The envelope-function equation has been solved exactly for both an homogeneous and a periodically modulated magnetic field perpendicular to the cylinder axis. The nature and energy dispersion of the quantum states reflects the interplay between different lengthscales, namely, the cylinder diameter, the magnetic length, and, possibly, the wavelength of the field modulation. We show that a transverse homogeneous magnetic field drives carrier states from a quasi-2D (cylindrical) regime to a quasi-1D regime where carriers form channels along the cylinder surface. Furthermore, a magnetic field which is periodically modulated along the cylinder axis may confine the carriers to tunnel-coupled stripes, rings or dots on the cylinder surface, depending on the ratio between the the field periodicity and the cylinder radius. Results in different regimes are traced to either incipient Landau levels formation or Aharonov-Bohm behaviour.
At low energy, electrons in doped graphene sheets behave like massless Dirac fermions with a Fermi velocity which does not depend on carrier density. Here we show that modulating a two-dimensional electron gas with a long-wavelength periodic potential with honeycomb symmetry can lead to the creation of isolated massless Dirac points with tunable Fermi velocity. We provide detailed theoretical estimates to realize such artificial graphene-like system and discuss an experimental realization in a modulation-doped GaAs quantum well. Ultra high-mobility electrons with linearly-dispersing bands might open new venues for the studies of Dirac-fermion physics in semiconductors.
The shakeup emission spectrum in a two-dimensional electron gas in a strong magnetic field is calculated analytically. The case of a localized photocreated hole is studied and the calculations are performed with a Nozieres-De Dominicis-like Hamiltonian. The hole potential is assumed to be small compared to the cyclotron energy and is therefore treated as a perturbation. Two competing many-body effects, the shakeup of the electron gas in the optical transition, and the excitonic effect, contribute to the shakeup satellite intensities. It is shown, that the range of the hole potential essentially influences the shakeup spectrum. For a short range interaction the above mentioned competition is more important and results in the shakeup emission quenching when electrons occupy only the lowest Landau level. When more than one Landau level is filled, the intensities of the shakeup satellites change with magnetic field nonmonotonically. If the interaction is long range, the Fermi sea shakeup processes dominate. Then, the satellite intensities smoothly decrease when the magnetic field increases and there is no suppression of the shakeup spectrum when the only lowest Landau level is filled. It is shown also that a strong hole localization is not a necessary condition for the SU spectrum to be observed. If the hole localization length is not small compared to the magnetic length, the SU spectrum still exists. Only the number of contributions to the SU spectrum reduces and the shakeup processes are always dominant.
The interaction between a single hole and a two-dimensional, paramagnetic, homogeneous electron gas is studied using diffusion quantum Monte Carlo simulations. Calculations of the electron-hole correlation energy, pair-correlation function, and the electron-hole center-of-mass momentum density are reported for a range of electron--hole mass ratios and electron densities. We find numerical evidence of a crossover from a collective Mahan exciton to a trion-dominated state in a density range in agreement with that found in recent experiments on quantum well heterostructures.
We study the spin Hall effect of a two-dimensional electron gas in the presence of a magnetic field and both the Rashba and Dresselhaus spin-orbit interactions. We show that the value of the spin Hall conductivity, which is finite only if the Zeeman spin splitting is taken into account, may be tuned by varying the ratio of the in-plane and out-of-plane components of the applied magnetic field. We identify the origin of this behavior with the different role played by the interplay of spin-orbit and Zeeman couplings for in-plane and out-of-plane magnetic field components.
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